It's rare, but it's perfectly and not that expensive, if it's worth it over the very common 34 given the tiny difference is up to you - spa cycles or xxcycle will sell the chainrings, they both generally have most stuff.

Tin Pot wrote:

I'm not familiar with Shimano or SRAM, but I'm under the impression they only go as short as 165, and as low as 34/32.

34x36 is perfectly feasible too, and yes Shimano and SRAM only go 165 I believe, but rotor go smaller, and there's plenty of shorter square taper options that take standard BCD's

Tin Pot wrote:

So I might get a degree lower or so, but does a 32 cassette over come the change to a 165 crank?

Just changing to 32 vs 27 is much bigger difference than 172.5 to 165, it'll still gain you about 10%
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It's rare, but it's perfectly and not that expensive, if it's worth it over the very common 34 given the tiny difference is up to you - spa cycles or xxcycle will sell the chainrings, they both generally have most stuff.

Tin Pot wrote:

I'm not familiar with Shimano or SRAM, but I'm under the impression they only go as short as 165, and as low as 34/32.

34x36 is perfectly feasible too, and yes Shimano and SRAM only go 165 I believe, but rotor go smaller, and there's plenty of shorter square taper options that take standard BCD's

Tin Pot wrote:

So I might get a degree lower or so, but does a 32 cassette over come the change to a 165 crank?

Just changing to 32 vs 27 is much bigger difference than 172.5 to 165, it'll still gain you about 10%

SRAM Red goes to 162.5. Rotor do 155.

No amount of 32 vs 27 is going to solve a biomechanical issue with a closed hip angle.
For various reasons I am also looking to move to a shorter crank to open the hip angle. Certainly on the TT bike and perhaps on the road and CX.
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I use a 170mm or 165mm crank, and to be honest can't really tell much difference, however, 172.5 immediately feels too long and 175 feels horrible. My inside leg is 780mm so using the fromula inside leg * 0.216 gives me a theoretical crank of 168.5mm

I see no difference in cadence, power or speed between 165 and 170mm cranks

In terms of gearing, I really don't notice, the difference between 165 and 170 is 3% the difference between a 27 and 28 tooth cassette is 3.7%. Even then I am not sure that the comparison is right, because the cadence between a short and long crank will be identical for any given speed, a longer crank has a slightly longer lever, but a shorter crank may give a slightly better pedal efficiency if you have short legs, so it is entirely possible that the efficiency and leverage improvement cancel each other out.

In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette

You may not notice any difference in cadence/speed/etc between say 172.5 & 165, however you'll be a gear out (can't remember if it is higher or lower) to match the output.
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In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette

I still cant figure out why the SRAM Red rear mech (it's an older 10sp version) on my TT bike wont accept a 28t cog. All the tech docs says it should, but no amount of b-limit screw adjustment will allow it to accept one.

I'm contemplating Wales later in the year, and if I use the TT, I really don't fancy having a 25t as my largest ring. Will be tempted to stick a long cage cheaper SRAM mech on it for that race, with maybe a 30+ cassette, just so I know I can keep spinning.

Relevant to Tin Pot's original question is factoring in any front mech restrictions. The same TT bike rubs badly on the smaller cogs if in the little ring on the front, as I put a set of compact rings on for another race last year to make up for the lack of anything more than 25t at the back. Semi-compact and standard double are fine. It is apparently a known restriction of the frame and that particular front mech that you can't cross-chain little-little on a 34. That's workable though, as I just switch to the big ring once it begins to rub.

In terms of cassette, most standard length derailleurs will take a 32 cassette, so you could get away with a compact chainset (50/34) and an 11-32 cassette

I still cant figure out why the SRAM Red rear mech (it's an older 10sp version) on my TT bike wont accept a 28t cog. All the tech docs says it should, but no amount of b-limit screw adjustment will allow it to accept one.

I'm contemplating Wales later in the year, and if I use the TT, I really don't fancy having a 25t as my largest ring. Will be tempted to stick a long cage cheaper SRAM mech on it for that race, with maybe a 30+ cassette, just so I know I can keep spinning.

Relevant to Tin Pot's original question is factoring in any front mech restrictions. The same TT bike rubs badly on the smaller cogs if in the little ring on the front, as I put a set of compact rings on for another race last year to make up for the lack of anything more than 25t at the back. Semi-compact and standard double are fine. It is apparently a known restriction of the frame and that particular front mech that you can't cross-chain little-little on a 34. That's workable though, as I just switch to the big ring once it begins to rub.